Ronald Hunter (1995) summarized the many hazards that mammalian spermatozoa encounter during their passage through the female reproductive tract (Figure 8.1). Most of the sperm perish, being engulfed by an army of leucocytes, or entrapped by mucus, so that they advance no further than the cervix, uterus or uterotubal junction. Some of them fail to undergo physiological changes, triggered by the female’s physiology, that are essential if they are to fertilize ova (e.g. capacitation, hyperactivation of sperm motility and the acrosome reaction). Hence, only a small fraction of male gametes that enter the female tract will reach her oviducts and, of these, only one will unite with each ovum to bring about fertilization (Table 8.1).